take care of initializing the labels array as late as possible

Octokit.Tests.Integration/Clients/IssuesLabelsClientTests.cs

@@ -41,7 +41,7 @@ public class IssuesLabelsClientTests : IDisposable
         Assert.Empty(issueLabelsInfo);
 
         var issueUpdate = new IssueUpdate();
-        issueUpdate.Labels.Add(label.Name);
+        issueUpdate.AddLabel(label.Name);
         var updated = await _issuesClient.Update(_repositoryOwner, _repository.Name, issue.Number, issueUpdate);
         Assert.NotNull(updated);
         issueLabelsInfo = await _issuesLabelsClient.GetForIssue(_repositoryOwner, _repositoryName, issue.Number);

Octokit/Models/Request/IssueUpdate.cs

@@ -60,5 +60,16 @@ namespace Octokit
                 return String.Format(CultureInfo.InvariantCulture, "Title: {0}",Title);
             }
         }
+
+        public void AddLabel(string name)
+        {
+            // lazily create the label array
+            if (Labels == null)
+            {
+                Labels = new List<string>();
+            }
+
+            Labels.Add(name);
+        }
     }
 }